Blasting cap



Sept. 23 1947. BURROWS 2,427,899

' BLAsTfme CAP.

Filed Sept. 4, 1940 LCZVVZO]? A.BZU'TOWJ INVENTOR ATTORNEY Patented Sept. 23, 1947 BLASTING CAP Lawton A. Burrows, Woodbury, N. J., asslgnor to E. I. du Pont de Nemours"& Company, Wilmington, Del., a corporation of Delaware a Application September 4, 1940, Serial No. 355,311 V 3 Claims. (Cl. 102-29) This invention relates to an improved blasting cap and more particularly to a cap adapted to be initiated by means of a fuse and possessing enhanced safety properties.

As is well known, detonators, or blasting caps, are employed to initiate the explosion f the main charges in blasting where dynamite or other high explosive is utilized. The caps contain, ordinalrlly, several difierent charges of varying degrees of sensitlveness all enclosed within a cylindrical metal shell. A base charge is first introduced of a compound, such as tetryl, having a lower order of sensitiveness to shock than the common primary detonatingcompounds. This base charge is suflicient in amount and energy content to bring about the complete detonation of the main explosive charge. Above the base charge is a primer charge of a faster and more sensitive explosive compound, such as mercury fulminate, lead azide, and the like. It is frequently desirable, especially when lead azide is used, to employ an ignition composition above the primer charge in order that initiation may be efl'ected with certainty by the spit of a fuse. In the case of fuse caps, the ignition top charge is exposed at the open end of the shell, prior to insertion of the fuse and crimping to the shell.

Since this ignition charge is of necessity readily ignitible by means of a fuse, it is usually capable also of ignition by chance sparks that may fall thereon; hence, a potential hazard exists in this exposed charge.

The object of the present invention is an improved blasting cap of the type adapted to be fired by means of ,a, fuse. A further object is such a cap which possesses increased safety properties and which is not explodable by contact of chance sparks with its top charge, A still further object is a cap of this type which does not detonate completely when it is initiated in the absence of confinement. Additional objects will be disclosed as the invention is described further hereinafter.

- I have found that the foregoing objects are accomplished, and the disadvantages of the prior art overcome, when I design the blasting cap assembly in accordance with the present invention. I introduce into the, cap, between the primer charge and the top ignition charge, an

intermediate charge comprising an explosive ma- 2 terial of such amount and of such degree of compression that it is incapable of transmitting the combustion from the ignition composition to the primer charge in the absence of confinement.

This intermediate charge, however,does transmit the ignition to the primer charge when'its combustion takes place under confinement, such, for example, as that provided by a fuse crimped into the cap shell in a position to ignite the top charge.

Various explosive compositions may be employed as theintermediate charge according to my invention. I find very advantageous a. layer of mannitol hexanitrate, commonly designated nitromannite, which has been loaded under a pressure of more that 25 lbs. I find the pressure range of 50 to 250 lbs. to comprise the limits within which efiective compression is attained. These pressures are the ones used with caps having an inside diameter of 0.218 inch, and it will be understood that this diameter cap is intended when pressures are stated. in this specification. The pressure range of 50 to 250 lbs, as applied, corresponds substantially to 1300-6500 lbs, per square inch. The nitromannite which has been subjected to the above pressure may be considered dead-pressed, that is to say, confined or unconfined, it will not detonate When'ignited by a small quantity of a relatively small ignition or cap charge, nor will it propagate combustion from the ignition to the primer charge unless confined; instead, the flame becomes extinguished in the absence of confinement. It will be understood, therefore, that sufilcient of the intermediate charge must be present to allow extinction of the combustion within said charge.

The invention is illustrated by the following specific embodiments of naps loaded in accordance therewith. These examples are not intended to limit the invention in any way.

Example 1 Fuse caps were prepared using (1) aluminum shells, (2) a S-grain base charge of tetryl pressed at 200 lbs., (3) a primer charge comprising 1.25 grains of a lead azide composition pressed at 150 lbs., and (4) an ignition charge consisting of a -45 blend of tetryl-lead styphnate pressed at lbs. ,An intermediate charge of nitromannite was introduced into the above caps between the functioning of primer and ignition compositions in. accordance with my invention, this charge beingpressed subsequent to the primer charge and ahead or the ignition charge. The following results show the efficacy of the intermediate, dead-pressed charge in preventing detonation of said cap by chance sparks, while not interfering with the proper the cap when initiated in the usual manner by means of the regular fuse:

Execution Inter ediate Char e G.Nitro- Prcs- Normal m mannit sure Initiation Spark Test Det. Fail. Det. Fail.

The foregoing results show the effectiveness of the intermediate charge of my invention in accomplishing the desired result. When an amount of as little as 0.4 grain of nitromannite was used, this was found to be insufficient to safeguard the cap. On the other hand, when as much as 2.0 grains was used, this was sufficient to blow the shell open and to, make the combination inoperative. Likewise, with respect to compression of the intermediate charge, a pressure of 25 lbs. was found insuflicient to dead-press the material, while pressures in excess of 250 lbs. were unsatisfactory.

The invention is graphically illustrated in the accompanying drawing, in which I represents a term "confinement "include such enclosure as results from the incylindrical aluminum shell into which the base charge 2 of tetryl has been pressed. The primer charge of lead azide 3 is represented here as contained within a cavity in the base charge. This location of the primer charge protects it from any accidental shock effects, but this charge might equally Well be located above the base charge entirely. Its position in this respect is no part of the present invention. Above the primer charge is located the intermediate charge of my invention 4, preferably nitromannite dead- -pressed under a pressure of between 50 and 250 lbs. The ignition charge 5, comprising a 55-45 blend of tetryl and lead styphnate is placed above the dead-pressed charge.

My invention is generally advantageous and practical with caps of the fuse type. It is not limited in any way to the charges cited in the foregoing examples. Any base charge employed satisfactorily in caps of the prior art may be used inthe caps, according to my invention. Similarly, it is not limited to caps containing lead azide as primer charge. In the examples, an ignition composition was used comprising a blend of lead styphnate and tetryl. The invention is not limited to these materials. I find the lead styphnate-tetryl compositions very satisfactory, however, as ignition agents, and prefer to use such compositions .within the range of 30-70 parts of tetryl and 70-30 parts of lead styphnate. Whatever the ignition composition used, it is important that it be one which is compatible with the shell wall material and that sufficient of it be used to initiate the intermediate layer satisfactorily, though not enough to rupture said shell.

Nitromannite is the preferred material for my intermediate dead-pressed charge, and I find the most satisfactory amount for use to be between 0.6 and 1.6 grains. It will be understood, however, that any other material may be used which possesses the ability to halt the combustion when confinement is absent and to propagate it to the primer charge under confinement. When the is used, this is intended to troduction of a fuse into the open end of the cap shell and all cases where the combustion gases are retained temporarily under pressure. The use of too large a charge of nitromannite defeats the purpose of the invention and destroys the confinement by bursting the upper portion of the shell prematurely. Other materials than nitromannite which may be used as intermediate charges include erythritol tetranitrate, nitrated sugar-nitrostarch mixtures, and smokeless powder.

My invention has been described fully in the foregoing, but it will be understood that many details of composition, properties, and assembly may be altered without departing from the spirit of the invention. I intend to be limited therefore only by the following patent claims.

I claim:

1. In a blasting cap of the fuse type comprising a metal shell, a base charge of a detonating explosive within the shell, a primer charg of lead azide above the base charge, and an ignition charge above said primer charge: a dead-pressed charge of mannitol hexanitrate intermediate between the ignition and primer charges.

2. In a blasting cap comprising a metal shell, a

base charge of a detonating explosive within said shell, and a primer charge of lead azide above said base charge: an ignition charge comprising a blend of tetryl and lead styphnate above the charge of lead=azide, and a charge intermediate between the ignition and primer charge comprising dead-pressed mannitol hexanitrate,

3. In a blasting cap comprising a metal shell,

a base charge of a detonating explosive within said shell, and a primer charge of lead azide above said base charge: an ignition charge comprising a blend of tetryl and lead styphnate above the charge of lead azide, and a dead-pressed charge intermediate between the ignition and primer charges, said charge being selected from the group consisting of nitromannite, erythritol tetrani'trate, nitrated sugar-nitrostarch mixtures,

and smokeless powder.

I LAWTON A. BURROWS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,928,208 Large Sept. 26, 1933 1,797,509 Lewis Mar. 24, 1931 1,744,693 Bergeim Jan. 21, 1930 1,438,431 Eschbach Dec. 12, 1922 2,207,137 Watts July 9, 1940 2,133,119 Smith Oct. 11, 1938 1,930,765 Meissner Oct. 17, 1933 1,906,869 Noddin May 2, 1933 FOREIGN PATENTS Number Country Date 16,405 Great Britain Oct. 26, 1914 

